The invention relates to a reception method in which received symbols are functions of bits, which are detected at successive trellis levels A and B by adding to a sum of a transition metric of each path that has led to each state of the level A a transition metric of a path branch relating to a bit 1 and a bit 0 and leading to the next level B and; by comparing at the level B that follows the level A the sums of the transition metrics of path branches entering each state; and by selecting, on the basis of the sums of the transition metrics, in each state of the level B the best of the two paths as a survivor path.
The invention also relates to a reception method in which received symbols are functions of bits, which are detected at successive trellis levels A and B by adding to a sum of a transition metric of a path that has led to each state of the level A a transition metric relating to a bit 1 and a bit 0 and leading to the next level; by comparing the sums of the transition metrics of path branches entering each state at the level B that follows the level A; and by selecting, on the basis of the sums of the transition metrics, in each state of the level B the best of two paths as a survivor path.
The invention further relates to a receiver, which comprises means for forming a transition metric of a trellis, the means being used for determining bits relating to received symbols; and selecting means for adding to the sum of the transition metric of each path that has led to each state of the level A, of the two successive levels A and B, a transition metric of two path branches leading to the next level B; for comparing at the level B that follows the level A the sums of the transition metrics of the path branches entering each state; and for selecting, on the basis of the sums of the transition metrics, in each state of the level B the best of the two paths as a survivor path.
The invention further relates to a receiver comprising means for forming the transition metric of the trellis, the means being used for determining the bits relating to the received symbols; and selecting means for adding to the sum of the transition metric of each path that has led to each state of the level A, of the levels A and B, the transition metric of the two path branches leading to the next level; for comparing the sums of the transition metrics of the path branches entering each state at the level B that follows the level A; and for selecting, on the basis of the sums of the transition metrics, in each state of the level B the best of the two paths as a survivor path.
In a finite-state, discrete-time Markov process, with white noise influencing the estimates usually made concerning the conditions of a digital cellular radio system, the Viterbi algorithm provides for sequence estimation an optimum recursive algorithm, which is not, however, equally good for detecting individual bits. A received signal can be Viterbi-decoded at a base station or at a subscriber terminal of a cellular radio system. In Viterbi-decoding of a receiver, the most probable sequence of transmitted data is searched for. Thus, in Viterbi-decoding, the symbols corresponding to the transmitted information are detected, the symbols representing the bits or bit combinations of the transmitted message. The Viterbi algorithm is used, as known, in signal detection and decoding. The Viterbi algorithm generates an ML (Maximum Likelihood) sequence estimate from a signal and typically also makes soft decisions for channel decoding functions. The ML estimate comprises estimates of the symbol sequences that the signal comprises. The ML method is discussed in Proakis, J., G., Digital Communications, McGraw-Hill Book Company, 1989, chapter 4.2 and the hardware implementation of the Viterbi algorithm is discussed in a publication by Fettweis, G., Meyer, H., High-Speed Parallel Viterbi Decoding: Algorithm and VLSI Architecture, IEEE Communications Magazine, Vol. 29(5), 1991, which are included herein as references.
A prior art Viterbi detector requires a correct path tracing phase for making a bit decision. This causes delay in the detection of the bits. In addition, the Viterbi detector falters, i.e. loses the correct path for the duration of several state transitions after having selected a wrong path branch even once.
An object of the present invention is thus to provide a reception method and a receiver similar to the Viterbi algorithm, the method and the receiver allowing direct bit decisions to be made, without employing the correct path tracing.
This is achieved with a method described in the preamble, characterized by generating at the level B, separately for each bit of a symbol, numbers relating to the bit 1 and bit 0 and representing a probability of the transition metrics of the survivor paths; separately summing the numbers relating to the bit 1 and the bit 0 of more than one state and representing the probability of the transition metric; generating logarithms of the sums; and generating a difference of the logarithms of the sums, a received signal being determined on the basis of the difference.
A method described in the preamble is further characterized by generating at the level B, separately for each bit in a symbol, numbers relating to the bit 1 and the bit 0 and representing the probability of the transition metric of the survivor paths; separately summing the numbers relating to the bit 1 and the bit 0 of more than one state and representing the probability of the transition metric; dividing the generated sums by each other; generating a logarithm of the quotient of the sums, a received bit being determined on the basis of the logarithm.
A receiver of the invention is characterized in that the receiver comprises means for generating at the level B, separately for each bit of a symbol, numbers relating to the bit 1 and the bit 0 and representing the probability of the transition metric of the survivor paths; means for separately summing at the level B the numbers relating to the bit 1 and the bit 0 of more than one state and representing the transition metric; means for storing the results of the summation of the numbers representing the probability of the transition metric; means for generating logarithms of said sums; and means for generating a difference of the logarithms of the sums, allowing thus a direct decision to be made on a received bit.
A receiver of the invention is further characterized in that the receiver comprises means for generating at the level B, separately for each bit of the symbol, the numbers relating to the bit 1 and the bit 0 and representing the probability of the transition metric of the survivor paths; means for separately summing the numbers relating to the bit 1 and the bit 0 of more than one state and representing the probability of the transition metric; means for storing the results of the summation of the numbers representing the probability of the transition metric; means for generating a quotient of the summation results of the numbers representing the probability of the transition metric formed by the means; and means for generating a logarithm of said divided sums to determine a received bit.
The method of the invention provides significant advantages: it enhances detection performance, speeds up bit decisions and removes faltering typical of a prior art Viterbi detection.